Optical amplification in coherence reflectometry

ABSTRACT

The present invention relates to an apparatus and a method for optical coherence reflectometry, in particular for optical coherence tomography. The invention particularly relates to the route of the light field in the sample arm. The reflected light field in the sample arm is amplified before being received by a combining means, said combining means being capable of receiving the reflected light field from the sample arm as well as the second reflected light field from the reference arm. Thereby, it is possible to direct substantially all light energy in the sample arm to the combining means, and to obtain fully the utilisation of the amplification of the reflected light field since preferably only the reflected light field is amplified by the optical amplifier. This leads to an improved signal-to-noise ratio (SNR) whereby an increase of the maximal penetration depth is obtained. Thereby, the apparatus is useful for obtaining optical biopsies of transparent as well as non-transparent tissues as well as new technical fields wherein the increased SNR allows the use of the present apparatus.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DK01/00573 which has an Internationalfiling date of Sep. 4, 2001, which designated the United States ofAmerica.

The present invention relates to an apparatus for optical coherencereflectometry, in particular for optical coherence tomography.

BACKGROUND

Optical low-coherence reflectometry (OLCR) is used for example foranalyzing inhomogeneities in optical waveguides and optical devices. Inthis method light is transmitted down the optical fibre and lightresulting from the interaction with an inhomogeneity in the opticalfibre is back-scattered. The light is split into two arms; a sample armand a reference arm. When the optical pathlength in the sample armmatches the pathlength in the reference arm coherent interference occursand the distance the light has travelled in the sample arm may bedetermined.

Optical low-coherence reflectometry is also used in the imaging of2-dimensional and 3 dimensional structures, eg. biological tissues, inthis respect often referred to as optical coherence tomography (OCT).OCT can be used to perform high-resolution cross-sectional in vivo andin situ imaging of microstructures, such as in transparent as well asnon-transparent biological tissue or other absorbing and/or random mediain generel. There are a number of applications for OCT, such asnon-invasive medical diagnostic tests also called optical biopsies. Forexample cancer tissue and healthy tissue can be distinguished by meansof different optical properties.

OLCR can be extended through the use of polarized light. The light fieldtowards the reference and sample is then polarized. After combining thelight reflected from the reference and the sample, the combined lightfield is split up again into two new light fields with perpendicularpolarization states. Through this method the birefringent properties ofthe sample can be investigated in addition to the information obtainablewith ordinary OLCR adding to the systems ability to discriminate betweencertain types of materials within the sample. This method also appliesto OCT often referred to as polarization sensitive OCT (PS-OCT).

In order to optimize optical low-coherence reflectometry measurementsand imaging various suggestions to increase signal-to-noise ratio (SNR)have been discussed in the art.

U.S. Pat. No. 5,291,267 (Sorin et al.) discloses optical reflectometryfor analyzing inhomogeneities in optical fibres. In U.S. Pat. No.5,291,267 amplification of the light reflected from the optical fibre isconducted. In particular U.S. Pat. No. 5,291,267 suggests to use thelight source as an amplifier in order to save costs.

WO 99/46557 (Optical Biopsies Technologies) discusses SNR in a systemwherein a reference beam is routed into a long arm of an interferometerby a polarizing beamsplitter. In general the reference suggest toinclude an attenuator in the reference arm to increase SNR. In abalanced setup the reference on the other hand suggests to increase thepower of the reference arm in order to increase SNR.

In “Unbalanced versus balanced operation in an optical coherencetomography system” Podoleanu, A. G., Vol. 39, No. 1, Applied optics,discussed various methods of increasing SNR in unbalanced and balancedsystems, respectively. Reduction of power in the reference arm wassuggested as well as reduction of fibre end reflections to increase theSNR.

The present invention relates to an optimisation of opticallow-coherence reflectometry whereby an increase of the SNR is obtainedleading to a better result of the measurements, in particular inrelation to penetration depth of the system, so that the penetrationdepth increases, when the SNR increases.

SUMMARY OF THE INVENTION

Thus, the present invention relates to an apparatus for opticalcoherence reflectometry comprising

-   -   a light source for providing a light signal    -   splitting means for dividing said light signal into a first        light field and a second light field,    -   means for directing the first light field to a sample, and means        for directing a first reflected light field from the sample,        wherein an optical amplifier is inserted in the first reflected        light field, said optical amplifier being different from the        light source, and means for directing the amplified first        reflected light field to a combining means, so that the        amplified first reflected light field is directed to the        combining means through another route than a route through the        splitting means for dividing the light signal,    -   means for directing the second light field to a reference path        comprising a reflecting means, and means for directing a second        reflected light field from the reference path to the combining        means,    -   combining means for receiving said amplified first reflected        light field and said second reflected light field to generate a        combined light signal, and    -   at least one detecting means for detecting the combined light        signal and outputting detection signals.

In the present context the term “optical coherence reflectometry” isused in its normal meaning, and thereby equivalent to “opticallow-coherence reflectometry, OLCR” and in particular the term meansoptical coherence tomography, OCT, and polarisation-sensitive opticalcoherence tomography, PS-OCT.

The present apparatus offers a better signal-to-noise ratio (SNR)whereby an increase of the maximal penetration depth is obtained.Thereby, the apparatus according to the present invention is especiallyuseful for obtaining optical biopsies of transparent as well asnon-transparent tissues.

In particular a combination of the arrangement of amplificationdiscussed above and reduction of fibre end reflections increases thesignal-to-noise ratio leading to an improved system.

The term “sample path” or “sample arm” is used to define the routetravelled by the light from the light source to the sample and reflectedfrom the sample to the combining means. In the present context the lightfield and routes relating to the sample arm is denoted the first lightfield and the first light route, respectively.

Correspondingly the term “reference path” or “reference arm” is used todefine the route travelled by the light from the light source to thereference reflection means and reflected from the reflection means tothe combining means. In the present context the light field and routesrelating to the reference arm is denoted the second light field and thesecond light route, respectively.

In another aspect the present invention relates to a method forproviding a result of a sample comprising

-   -   establishing a light source for providing a light signal,    -   splitting said light signal into a first light field and a        second light field,    -   directing the first light field to a sample, and the second        light field to a reference path,    -   receiving the first reflected light field from the sample,        optically amplifying the first reflected light field, and        directing the first reflected light field in a combining means,    -   receiving the second reflected light field,    -   combining said amplified first reflected light field and said        second reflected light field to generate a combined light        signal,    -   detecting the combined light signal obtaining detection signals,        and    -   processing the detection signals obtaining the result of the        sample.

In the present context the term “result of the sample” refers in OCT tothe image of the tissue obtained. When using the present invention inOLCR in optical fibres used for example in the communication technologythe result relates to the signal obtained, such as a signal relating tothe distance to an inhomogeneity in the device under test.

DRAWINGS

FIG. 1 shows an unbalanced conventional OCT system according to priorart, wherein an attenuator has been inserted in the reference arm.

FIG. 2 shows a balanced apparatus according to the invention, whereinthe amplified first reflected light field is directed to the combiningmeans through another route than a route through the splitting means. Ay-coupler is inserted in the sample arm to receive the reflected lightfield from the sample.

FIG. 3 shows the balanced detection means of FIG. 2 in detail.

FIG. 4 shows a balanced apparatus as in FIG. 2 wherein an opticalcirculator has been inserted instead of the beam splitting means in thesample.

FIG. 5 shows a balanced system according to prior art chosen asreference system. The system is similar to the system shown in FIG. (2)except for omittion of the optical amplifier and the y-coupler in thesample part. The y-coupler is omitted since it is no longer necessaryfor the light to follow a different path to and from the sample.

FIG. 6 shows the optimum splitter ratio for the system shown in FIG. (2)investigated in the absence of an optical amplifier, i.e. theamplification factor is set to 1 and the optical noise added from theamplifier is set to zero. The SNR of the system is compared to thereference system, where both systems are used in the uncoated case.

FIG. 7 shows the optimum splitter ratio for the system shown in FIG. (2)investigated in the absence of an optical amplifier, i.e. theamplification factor is set to 1 and the optical noise added from theamplifier is set to zero. The SNR of the system is compared to thereference system, where both systems are used in the coated case.

FIG. 8 shows the effect of including an optical amplifier on the novelsystem shown in FIG. (2). The SNR of the novel system is compared to thereference system FIG.(5), where both systems are used in the uncoatedcase.

FIG. 9 shows the effect of including an optical amplifier on the novelsystem shown in FIG. (2). The SNR of the novel system is compared to thereference system FIG.(5), where both systems are used in the coatedcase.

FIG. 10 shows the optimum splitting ratio for the novel system shown inFIG. (2), with the optical amplifier set at a fixed amplification factorof 20 dB. The SNR of the novel system is compared to the referencesystem FIG.(5), where both systems are used in the uncoated case.

FIG. 11 shows the optimum splitting ratio for the novel system shown inFIG. (2), with the optical amplifier set at a fixed amplification factorof 20 dB. The SNR of the novel system is compared to the referencesystem FIG.(5), where both systems are used in the coated case.

FIG. 12 shows the relative SNR shown as function of the thermal noisefor the system shown in FIG. (4), where the splitting ratio is set tothe optimum setting and Γ_(incoh) is taken as the coated case.

FIG. 13 shows a balanced apparatus according to the invention, whereinonly the first reflected light field is amplified by the opticalamplifier and hereafter directed to the combining means.

FIG. 14 shows the SNR of the system shown in FIG. (5) as a function ofthe splitting ratio x/(1−x) in the uncoated case. The optimum splittingratio, for the set of parameter values chosen as an example, is found tobe 33.4/66.6, and for the uncoated case, this splitting ratio is used inthe reference system.

FIG. 15 shows the SNR of the system shown in FIG. (5) as a function ofthe splitting ratio x/(1−x) in the coated case. The optimum splittingratio for the set of parameter values chosen as an example is found tobe 33.4/66.6, and for the coated case, this splitting ratio is used inthe reference system.

FIG. 16 shows the effect of the optical amplifier in the novel system inFIG. (13) as a function of amplification factor. A splitting ratio of50/50 has been chosen for the novel system whereas the optimum splittingratio is used for the reference system. The novel system in FIG. (13)and the reference system FIG. (5) are both used in the uncoated case.Relative SNR refers to the SNR of the novel system divided by theoptimum SNR of the reference system.

FIG. 17 shows the effect of the optical amplifier in the novel system inFIG. (13) as a function of amplification factor. A splitting ratio of50/50 has been chosen for the novel system whereas the optimum splittingratio is used for the reference system. The novel system in FIG. (13)and the reference system FIG. (5) are both used in the coated case.Relative SNR refers to the SNR of the novel system divided by theoptimum SNR of the reference system.

FIG. 18 shows the SNR of the novel system shown in FIG. (13), relativeto the optimum reference system, as a function of the splitting ratiox/(1−x) in the uncoated case. The optical amplifier is set at a fixedamplification factor of 20 dB. The optimum splitting ratio for the setof parameter values chosen as an example is found to be 45.1/54.9.

FIG. 19 shows the SNR of the novel system shown in FIG. (13), relativeto the optimum reference system, as a function of the splitting ratiox/(1−x) in the uncoated case. The optical amplifier is set at a fixedamplification factor of 20 dB. The optimum splitting ratio for the setof parameter values chosen as an example is found to be 45.1/54.9.

FIG. 20 shows the SNR of the novel system shown in FIG. (4), relative tothe optimum reference system, as a function of the splitting ratiox/(1−x) in the uncoated case. The optical amplifier is set at a fixedamplification factor of 20 dB. The optimum splitting ratio for the setof parameter values chosen as an example is found to be 73.3/26.7.

FIG. 21 shows the SNR of the novel system shown in FIG. (4), relative tothe optimum reference system, as a function of the splitting ratiox/(1−x) in the coated case. The optical amplifier is set at a fixedamplification factor of 20 dB. The optimum splitting ratio for the setof parameter values chosen as an example is found to be 73.5/26.5.

FIG. 22 shows the relative SNR of the novel system shown in FIG. (4),relative to the optimum reference system, as a function of the noisecontribution of the receiver. The system is used in the coated case andthe splitting ratio is set to the optimum found in FIG. (21).

DESCRIPTION OF THE DRAWINGS

In FIG. 1 an unbalanced detection scheme, not according to theinvention, is shown for comparison reasons. The optical coherence systemis denoted 1. A light source 2 provides a light signal that is directedto a splitting means 3 for dividing said light signal into a first lightfield 4 and a second light field 5. The splitting ratio in FIG. 1 is setto 50/50. The first reflected light 9 and the second reflected light 10is combined by the splitter means 3 and a combined signal is directed tothe detector 8. The second reflected light field reflected from thereflection means 6 is attenuated by attenuator 7.

In FIG. 2 a detection scheme according to the invention is depicted. Theoptical coherence system is denoted 1. A light source 2 provides a lightsignal that is directed to a splitting means 3 for dividing said lightsignal into a first light field 4 and a second light field 5. Thesplitting ratio may be set to any suitable ratio, exemplified by theratio x/1−x. The first reflected light field 9 is directed to a balanceddetection means 11 comprising a combining means. An amplifier 12 isinserted in the first reflected light field to amplify the light signalreflected from the sample. A directing means 16 is inserted in the firstlight field 4 for directing the light field 4 to the sample and directthe reflected light field 9 to the optical amplifier 12. The secondreflected light field 10 reflected from the reflection means 6 is alsodirected to the balanced detection means 11 comprising a combiningmeans. The reflection means 6 is shown as a so-called corner cubeconfiguration.

The balanced detection means 11 is shown in detail in FIG. 3 comprisinga combining means 13, exemplified by a splitter having a splitting ratioof 50/50, capable of splitting the combined signal into first splitsignal 14 and second split signal 14′. The split signals 14, 14′ aredirected to the detectors 8, 8′ respectively. The detected signal may beoutput via 15 to a printing means, a display and/or a storage means.

In FIG. 4, a refinement of the novel system in FIG. (2) is shown. Toavoid the reduction of the first reflected light field 9 by thesplitting means 3, an optical circulator 16 is inserted to directsubstantially all the light power in the first reflected light field 9to the optical amplifier 12.

In FIG. 5 a reference system used for comparison reasons in the examplesis shown. The reference system is according to prior art with noapplication implemented. The optical coherence system is denoted 1. Alight source 2 provides a light signal that is directed to a splittingmeans 3 for dividing said light signal into a first light field 4 and asecond light field 5. The splitting ratio may be set to any suitableratio, exemplified by the ratio x/(1−x). The first reflected light field8 is directed back to the splitting means 3. After the splitter, thefirst reflected light field 9 is—due to the nature of the splitterused—reduced by the factor (1−x) and directed to to the detection means11 comprising a combining means. The second reflected light field 10reflected from the reflection means 6 is also directed to the balanceddetection means 11 comprising a combining means. The reflection means 6is shown as a so-called corner cube configuration.

In FIG. 13 a detection scheme according to the invention is depicted.The optical coherence system is denoted 1. A light source 2 provides alight signal that is directed to a splitting means 3 for dividing saidlight signal into a first light field 4 and a second light field 5. Thesplitting ratio may be set to any suitable ratio, exemplified by theratio x/(1−x). The first reflected light field 8 is directed back to thesplitting means 3. After the splitter, the first reflected light field 9is—due to the nature of the splitter used—reduced by the factor (1−x)and directed to the optical amplifier 12, and thereafter to thedetection means 11 comprising a combining means. The second reflectedlight field 10 reflected from the reflection means 6 is also directed tothe balanced detection means 11 comprising a combining means. Thereflection means 6 is shown as a so-called corner cube configuration.

FIGS. 6-12 and 14-22 shows graphs relating to the examples below.

DETAILED DESCRIPTION

The present invention relates to an apparatus for optical coherencereflectometry, in particular optical coherence tomography.

One important aspect of the present invention is the route of the lightfield in the sample arm. The first reflected light field is amplifiedbefore being received by a combining means, said combining means beingcapable of receiving the first reflected light field from the sample armas well as the second reflected light field from the reference arm. Theamplified first reflected light field is directed to the combining meansthrough another route than a route through the splitting means fordividing the light signal from the light source into the sample arm andthe reference arm, respectively. Thereby, it is possible to directsubstantially all light energy from the first reflected light field tothe combining means, and to obtain fully the utilisation of theamplification of the first reflected light field. In other words by thepresent invention the amplified first reflected light field is directedto the combining means, so that only the reflected light field isamplified by the optical amplifier.

Another important aspect of the invention is that the optical amplifierinserted in the first reflected light field is different from the lightsource so that the effect of the light source may be regulatedindependent of the degree of amplification. In particular when using theapparatus in OCT certain safety regulations for the power densitytowards the sample has to be observed to reduce the risk of damages tothe sample under examination, such as biological tissue.

Light Source

The light source should be a broad band source and may be chosen fromone of the following categories:

-   -   continuous wave source, as for example superluminescent laser        diodes or other white light sources,    -   pulsed lasers, as for example femtosecond lasers,

The shape of the spectrum is important because the signal received—froma single reflection—at the detector as the reference beam optical pathlength is varied is the autocorrelation of the source spectrum.Therefore, the wider the source spectrum the narrower signal is receivedas the reference beam optical path length is varied leading to a higherspatial resolution.

The wavelength of the light source is adjusted to the purpose of theanalysis performed with the apparatus. The wavelength is mostly selectedin the range from 500 nm to 2000 nm. For non-transparent solid tissuethe wavelength is normally selected in the range from 1250 nm to 2000nm.

For retinal examinations the wavelength is mostly selected in the rangefrom 600 nm to 1100 nm.

Balanced/Unbalanced System

In general the system or apparatus according to the invention may beconstructed as either an unbalanced system or a balanced system. Theterms unbalanced and balanced are used in the normal meaning, ie. anunbalanced system refers to a system having one detecting means, whereasa balanced system refers to a system having two detecting means, whereineach detector receives signals from the sample arm as well as from thereference arm. In a balanced system the signals from the two detectorsare subtracted from each other in order to obtain the result.

Also a double balanced system may be used in the apparatus according tothe invention, a double balanced system referring to a system comprisingfour detecting means.

Noise

The noise in the apparatus or system according to the invention is thetotal sum of noise sources in the following parts:

-   -   Optical noise, such as noise from the light source and noise        from the optical amplifier.    -   Receiver noise, such as thermal fluctuations in the electronic        parts and shot noise.

The optical noise from the light source is manifested as the intensitynoise relating to the first reflected light and intensity noise relatingto the second reflected light as well as intensity noise from a mixtureof both.

In the following calculations, a specific configuration for the balanceddetection scheme applied to low coherent reflectometry has been chosen.However, the added benefit of introducing an optical amplifier withrespect to the signal-to-noise-ratio also applies to other realizationsof balanced detection and double balanced detection schemes.

For the following calculation a balanced detector system is assumedcomprising of two detectors and a fiber-optic splitter as shown in FIG.3.

The receiving device is assumed to receive three light fields, see FIG.3: From the reference arm the field is E_(reference)(t) having theintensity I_(reference), from the reflection being measured the field isE_(signal)(t) with intensity I_(signal), and from the sum of all light,which does not have a matching pathlength with the reference and thuswill not be temporally coherent with the reference, the field isE_(Inoch)(t) with the intensity I_(Incoh). Such light could stem fromall reflections from the sample except the reflection being measured andfrom undesired reflections from fiber ends, lenses or other opticalcomponents or devices. Noise contributions from an optical amplifier mayalso be included in I_(incoh).

Assuming that the coupler used in the balanced detector, see FIG. 3, issymmetric, the field incident on each detector 1 and detector 2respectively can be written as $\begin{matrix}{{\begin{bmatrix}{E_{1}(t)} \\{E_{2}(t)}\end{bmatrix} = {{{\mathbb{e}}^{{\mathbb{i}}\quad\varphi_{r}}\begin{bmatrix}a & {b\quad{\mathbb{e}}^{i\quad\varphi}} \\{b\quad{\mathbb{e}}^{i\quad\varphi}} & a\end{bmatrix}}\begin{bmatrix}{{E_{incoh}(t)} + {E_{signal}(t)}} \\{E_{reference}\quad(t)}\end{bmatrix}}},} & (1)\end{matrix}$where φ_(r) and φ expresses phase changes due to the coupler, and a andb are coupling constants. It is known from the art that if the coupleris assumed lossless this constraint will mean that a²+b²=1 and φ=±π/2.For a 50/50 coupler a=b=1/√2. Thus for the balanced detector theincident fields are: $\begin{matrix}{{\begin{bmatrix}{E_{1}(t)} \\{E_{2}(t)}\end{bmatrix} = {{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & {\mathbb{e}}^{i\quad{\pi/2}} \\{\quad{\mathbb{e}}^{i\quad{\pi/2}}} & 1\end{bmatrix}}\begin{bmatrix}{{E_{incoh}(t)} + {E_{signal}(t)}} \\{E_{reference}\quad(t)}\end{bmatrix}}},} & (2)\end{matrix}$where φ=π/2 has been chosen and the common phase change φ_(r) has beenassumed zero without loss of generality. Using this it is straightforward to calculate the electrical current i ₁ and i ₂ in each detectordue to a square law detection of the incident light power:$\begin{matrix}{{\begin{bmatrix}{i_{1}(t)} \\{i_{2}(t)}\end{bmatrix} = {\frac{1}{2}\eta\frac{e}{hv}\left\{ {\left\lbrack \quad\begin{matrix}{{I_{incoh}(t)} + {I_{signal}(t)} + {I_{reference}(t)}} \\{{I_{incoh}(t)} + {I_{signal}(t)} + {I_{reference}(t)}}\end{matrix} \right\rbrack + \left\lbrack \quad\begin{matrix}{2\quad{{Re}\left\lbrack {{{E_{incoh}(t)}{E_{signal}^{*}(t)}} + {{E_{signal}(t)}E_{reference}^{*}\quad(t){\mathbb{e}}^{i\quad{\pi/2}}} + {{E_{incoh}(t)}E_{reference}^{*}\quad{\mathbb{e}}^{i\quad{\pi/2}}}} \right\rbrack}} \\{2\quad{{Re}\left\lbrack {{{E_{incoh}(t)}{E_{signal}^{*}(t)}} + {{E_{signal}(t)}E_{reference}^{*}\quad(t){\mathbb{e}}^{{- i}\quad{\pi/2}}} + {{E_{incoh}(t)}E_{reference}^{*}\quad{\mathbb{e}}^{{- i}\quad\pi}}} \right\rbrack}}\end{matrix}\quad \right\rbrack} \right\}}}\quad,} & (3)\end{matrix}$where e is the electron charge, h Planck's constant, ν the averagewavelength of the light source, η the quantum efficiency of thephotodetectors, and Re[ ] denotes taking the real part of the argumentinside the brackets. Since the balanced detector detects the differencebetween the two currents, the received electrical signal i(t) becomes:$\begin{matrix}{{i(t)} = {{{i_{1}(t)} - {i_{2}(t)}} = {\eta\frac{e}{hv}2{{Re}\left\lbrack {{{{iE}_{signal}(t)}{E_{reference}^{*}(t)}} + {{{iE}_{incoh}(t)}{E_{reference}^{*}(t)}}} \right\rbrack}}}} & (4)\end{matrix}$

The first term is the signal due to the reflection to be measured andthe second term gives rise to the so-called beat noise which was studiedby K. Takada (“Noise in Optical Low-Coherence Reflectometry”) IEEE J. ofQuant. Electronics JQE-34 (7), 1098 (1998)).

The noise contributions are all expressed as the received noise powerafter electrical subtraction of the two signals received by eachphotodetector, measured per unit bandwidth of the receiving circuit.

Takada found that the beat-noise received, due to I_(incoh) in theabsence of an optical amplifier is given by: $\begin{matrix}{{\left\langle {\Delta\quad i_{beat}^{2}} \right\rangle = {\left( {\eta\frac{e}{hv}} \right)^{2}\frac{2}{\delta_{v}}\left\langle I_{reference} \right\rangle\left\langle I_{incoh} \right\rangle}},} & (5)\end{matrix}$where δv [Hz] is the effective line-width of the light source used. Tosimplify the following calculations we assume that the optical amplifierimposes no spectral distortion on the first reflected light field andthat the bandwidth of the optical amplifier is identical to that of thelight source.

It is common knowledge within the art that the so-called shot-noise dueto the particle nature of the photon-to-electron conversion in thephotodetectors is given by $\begin{matrix}{\left\langle {\Delta\quad i_{shot}^{2}} \right\rangle = {2{e\left( {\eta\frac{e}{hv}} \right)}{\left( {\left\langle I_{signal} \right\rangle + \left\langle I_{reference} \right\rangle + \left\langle I_{incoh} \right\rangle} \right).}}} & (6)\end{matrix}$

The photodetectors also have an inherent noise contribution, which isindependent of the incident light power. There are two contributions tothis noise: Thermal noise in the electrical circuit of the detectors andshot-noise due to dark current in photodetector. The reciever noise is:$\begin{matrix}{{\left\langle {\Delta\quad i_{reciever}^{2}} \right\rangle = {{\left\langle {\Delta\quad i_{thermal}^{2}} \right\rangle + \left\langle {\Delta\quad i_{dark}^{2}} \right\rangle} = {2\left( {{\frac{4k_{B}T}{R}F_{n}} + {2e\left\langle i_{dark} \right\rangle}} \right)}}},} & (7)\end{matrix}$

where k_(B) is Boltzman's constant, R the load resistance of each of thedetectors, T the temperature, F_(n) the noise figure of the electricalcircuit of the detectors normally dominated by the preamplifier,(i_(dark)) the dark current in each detector and the factor of 2 is dueto having two independent detectors. However, this receiver noise,independent of the incident light, is often more conveniently measuredexperimentally.

Thermal fluctuations in the electronic parts are independent of theamount of light used, and furthermore, the thermal fluctuations may bereduced by cooling of the detectors, and also by optimising theconstruction.

Shot noise relates to the particle nature of the light. The shot noiseis proportional to the amount of light received.

When inserting an optical amplifier in the apparatus it is inherent thatin addition to the amplification of the signal desired the optical noisewill inevitably also be amplified.

In this calculation an optical amplifier is modelled to amplify theincoming light and add optical noise due to intrinsic amplifier noise.Hence, the light intensity emitted from an optical amplifier is given by<I _(out) >=χ<I _(in) >+<I _(noise)>,  (8)where χ is the amplification factor, I_(in) the intensity of theincident light, and I_(noise) the intensity due to the intrinsicamplifier noise. For simplicity it is assumed that the optical bandwidthof I_(noise) equals that of the light source of the system, and that allwavelengths of I_(in) is amplified by the same factor.

Assuming that the pathlength of the reference light is perfectly matchedto that of the signal light being reflected by the reflection to bemeasured, the signal power received by the balanced detector is given by$\begin{matrix}{{\left\langle i_{signal}^{2} \right\rangle = {\left( {\eta\frac{e}{hv}} \right)^{2}\left\langle I_{reference} \right\rangle\left\langle I_{signal} \right\rangle}},} & (9)\end{matrix}$where it is assumed that the source is unpolarized. If polarized lightis used and the reference and signal light is made to match exactly, thesignal power i² _(signal) signal in Eq. (9) should be multiplied by afactor 2.

Using the above equations it is straight forward to derive thesignal-to-noise ratio (SNR): $\begin{matrix}\begin{matrix}{{SNR} = \frac{\left\langle i_{signal}^{2} \right\rangle}{B\left( {\left\langle {\Delta\quad i_{reciever}^{2}} \right\rangle\left\langle {\Delta\quad i_{shot}^{2}} \right\rangle\left\langle {\Delta\quad i_{beat}^{2}} \right\rangle} \right.}} \\{{= \frac{\alpha^{2}\left\langle I_{reference} \right\rangle\left\langle I_{signal} \right\rangle}{B\left\lbrack {\left\langle {\Delta\quad i_{reciever}^{2}} \right\rangle + {2e\quad{\alpha\left( {\left\langle I_{signal} \right\rangle + \left\langle I_{reference} \right\rangle + \left\langle I_{incoh} \right\rangle} \right)}} + {2{\alpha^{2}/\delta_{v}}\left\langle I_{reference} \right\rangle\left\langle I_{incoh} \right\rangle}} \right\rbrack}},}\end{matrix} & (10)\end{matrix}$where B is the effective bandwidth of the electrical detector system,and α=ηe/hν the responsivity of the detector.

Below the individual terms in Eq. (10) according to the configuration inFIG. (2) are specified.

If the light source emits the light intensity (I_(source)) then thelight intensity towards the receiver from the reference will be<I _(reference)>=(1−x)β<I _(source)>,  (11)where X is the coupling ratio towards the sample of the first couplerfrom the source, and β is a factor describing the loss of power as thelight is coupled out of the fiber, reflected (in this case from amovable retroreflector), and coupled back into the fiber. Forsimplicity, and without loss of generality, the factor β is set tounity.

For the equations mentioned below the equations 12′-16′ related to asystem of FIG. 2 and 12-16 relate to a system of FIG. 13. Thus, in afirst embodiment the light power incident upon the optical amplifier is

 <I _(in) >=x(1−x)(Γ+Γ_(incoh))<I _(source)>  (12)

where Γ is the reflectivity of the reflection to be measured, andΓ_(incoh) the sum of all other reflectivites from the sample and frome.g. a collimating or focusing lens guiding the light from the fiber tosample and back into the fiber. Note that the each individualreflectivity should be reduced by the applicable coupling loss to andfrom the fiber. The light power incident on the balanced detector fromthe optical amplifier will then be<I _(out) >=χx(1−x)(Γ+Γ_(incoh))<I _(source) >+<I _(noise)>  (13)

Thus the light power incoherent to the reference becomes<I _(incoh) >=χx(1−x)Γ_(incoh) <I _(source) >+<I _(noise)>  (14)and the light power related to the desired signal due to thereflectivity Γ becomes<I _(signal) >=χx(1−x)Γ<I _(source)>  (15)

Inserting Eq. (11), Eq.(14) and Eq.(15) into Eq (10) yields$\begin{matrix}{{SNR} = \frac{\alpha^{2}\chi\quad{x\left( {1 - x} \right)}^{2}\Gamma}{B\left\lbrack {\frac{\left\langle {\Delta\quad i_{receiver}^{2}} \right\rangle}{\left\langle I_{source} \right\rangle^{2}} + {2e\quad{\alpha\left( {\frac{\left( {1 - x} \right)\left( {1 + {\chi\quad{x\left( {\Gamma + \Gamma_{incoh}} \right)}}} \right)}{\left\langle I_{source} \right\rangle} + \frac{\left\langle I_{noise} \right\rangle}{\left\langle I_{source} \right\rangle^{2}}} \right)}} + {\frac{2\alpha^{2}}{\delta_{v}}\left( {{\chi\quad{x\left( {1 - x} \right)}^{2}\Gamma_{incoh}} + \frac{\left\langle I_{noise} \right\rangle}{\left\langle I_{source} \right\rangle}} \right)}} \right\rbrack}} & (16)\end{matrix}$

In another embodiment the light power incident upon the opticalamplifier is $\begin{matrix}{{\left\langle I_{in} \right\rangle = {\frac{1}{4}{x\left( {\Gamma + \Gamma_{incoh}} \right)}\left\langle I_{source} \right\rangle}},} & \left( 12^{\prime} \right)\end{matrix}$where Γ is the reflectivity of the reflection to be measured, andΓ_(Incoh) the sum of all other reflectivites from the sample and frome.g. a collimating or focusing lens guiding the light from the fiber tosample and back into the fiber. Note that the each individualreflectivity should be reduced by the applicable coupling loss to andfrom the fiber. The light power incident on the balanced detector fromthe optical amplifier will then be $\begin{matrix}{{\text{〈}I_{out}\text{〉}} = {\chi\frac{1}{4}{x\left( {\Gamma + \Gamma_{incoh}} \right)}\text{〈}I_{source}{\text{〉}.}}} & \left( 13^{\prime} \right)\end{matrix}$

Thus the light power incoherent to the reference becomes $\begin{matrix}{{\text{〈}I_{incoh}\text{〉}} = {{\chi\frac{1}{4}x\quad\Gamma_{incoh}\text{〈}I_{source}\text{〉}} + {\left\langle I_{noise} \right\rangle.}}} & \left( 14^{\prime} \right)\end{matrix}$and the light power related to the desired signal due to thereflectivity Γ becomes $\begin{matrix}{\left\langle I_{signal} \right\rangle = {\chi\frac{1}{4}x\quad\Gamma{\left\langle I_{source} \right\rangle\quad.}}} & \left( 15^{\prime} \right)\end{matrix}$

Inserting Eq. (11), Eq.(14) and Eq.(15) into Eq (10) yields$\begin{matrix}{{SNR} = \frac{\frac{1}{4}\left( {1 - x} \right)\chi\quad x\quad\Gamma}{B\left\lbrack {\frac{{\text{〈}\Delta\quad i_{reclever}^{2}\text{〉}} + {2{e\alpha}\text{〈}I_{noise}\text{〉}}}{\alpha^{2}\text{〈}I_{source}\text{〉}^{2}} + {\frac{1}{2}{\mathbb{e}}\frac{{\chi\quad{x\left( {\Gamma + \Gamma_{incoh}} \right)}} + {4\left( {1 - x} \right)\left( {1 + {{\alpha/e}\quad\delta_{v}\text{〈}I_{noise}\text{〉}}} \right)}}{\alpha\text{〈}I_{source}\text{〉}}} + {\frac{1}{2\delta_{v}}\left( {1 - x} \right)x\quad{\chi\Gamma}_{incoh}}} \right\rbrack}} & \left( 16^{\prime} \right)\end{matrix}$

In analogy with this derivation, the SNR can be derived for a widevariety of low-coherent reflectometer systems with balanced detectionand thus the performance of such systems can be easily compared.

Splitting Means

The general principle of OLCR and OCT is that distance travelled by thelight in the sample arm is correlated to the distance travelled by thelight in reference arm.

The light is emitted from a light source as discussed above and dividedinto a first light field and a second light field by a splitting means.The splitting means may be any means suitable for splitting a lightsignal into two light fields. The splitting means may be selected from abulk optic splitting means, a fibre optic splitting means, a holographicoptical element or a diffractive optical element.

In one embodiment the apparatus according to the invention comprises asplitting means capable of dividing the light signal into the sample armand the reference arm with a splitting ratio of the splitting meansbeing substantially 50%/50%.

However the present inventors have found due to the location of theamplifier as well as the route of the first reflected light field that afurther increase in SNR may be obtained when using a changeablesplitting ratio, so that from 1% to 99% of the light energy from thelight source is directed to the sample arm. It is preferred that morethan 50% of the light energy is directed to the sample, such as from 50%to 99% of the light energy from the light source is directed to thesample arm, such as from 55% to 90% of the light energy from the lightsource is directed to the sample arm, such as from 60% to 85% of thelight energy from the light source is directed to the sample arm, suchas from 65% to 85% of the light energy from the light source is directedto the sample arm.

In another embodiment it is preferred that from 1% to 60% of the lightenergy from the light source is directed to the sample arm, such as from20% to 55% of the light energy from the light source is directed to thesample arm, such as from 30% to 50% of the light energy from the lightsource is directed to the sample arm, such as from 40% to 50% of thelight energy from the light source is directed to the sample arm.

Sample Arm—First Light Field Route

The apparatus according to the invention comprises means for directingthe first light field to the sample. In a preferred embodiment at leasta part of the means for directing the first light field to the samplecomprises an optical fibre, so that the means in total comprises anoptical fibre and an optical system. An optical system may be includedfor focusing the first light field to the sample. The optical system forexample being one or more lenses.

It is preferred that the first light field is directed to the samplewithout being amplified. Thereby the intensity of the first light fieldonto the sample is exclusively determined by the light source. Thisleads to a better control of the light intensity in the sample arm,since the light directed to the sample conforms to the practical limitsfor sample light, such as an upper limit for the intensity to avoiddamages to the sample, and the light reflected from the sample may beamplified to the degree necessary for the SNR to be suitably increased.Thus, the amplifier is preferably located in a part of the sample arm bywhich only the reflected light field is traveling. This may beaccomplished by inserting a splitting means or a circulator to receivethe reflected first light field from the sample, or by inserting theoptical amplifier after the first reflected light field has passed thesplitting means used to split the light into the first and second lightfields.

In a preferred embodiment a circulator is inserted whereby substantiallyall light energy reflected from the sample is directed as the firstreflected light field to the optical amplifier.

The term light field as used herein means light field as normally usedfor the light in optical fibres, but does also include a light beam asnormally used in bulk systems and in the optical system.

Scanning Head

The sample is scanned by means known in the art, such as galvanometerscanners, polygon mirrors, resonant scanners, a scanning head.

Amplifier

Any optical amplifier suitable for amplifying the reflected first lightfield may be interposed in the light route from the sample to thecombining means. The amplifier may thus be a semiconductor, a resonantamplifier or a fibre and/or Raman amplifier. The amplification factormay be in the range from 1.5 to 1,000,000 times, such as from 20 to500,000 times, for example from 20 to 100,000 times, such as from 20 to50,000 times, such as from 20 to 10,000 times, such as from 20 to 1000times, such as from 20 to 100 times.

Reference Arm—Second Light Route

The apparatus according to the invention also comprises means fordirecting the second light field to the reflecting means. In a preferredembodiment at least a part of the means for directing the second lightfield to the reflecting means comprises an optical fibre, so that thedirecting means in total comprises an optical fibre and an opticalsystem. The optical system may be used for directing the second lightfield to the reflecting means, such as any kind of lenses, gratingsetc., known to the person skilled in the art.

Attenuation of the reflected second light field may be useful when usingan unbalanced system, whereas attenuation of the reference arm does notadd anything further to the SNR in a balanced system.

In a preferred embodiment the reflected second light field does not passthe splitting means for dividing the light signal when travellingtowards the combining means. It is an advantage to maintain as much aspossible of the second reflected light field on the route to thecombining means. This may be accomplished by inserting a circulator toreceive the second reflected light field from the reflection means todirect the second reflected light field directly to the combining means.

In a preferred embodiment a circulator is inserted to receive the secondreflected light field whereby substantially all light energy reflectedfrom the reflecting means is directed as the second reflected lightfield to the combining means.

The reflecting means may be any means suitable for reflecting the lightin the reference arm or means having a similar function, the function ofthe reference arm being its capability of allowing light to travel anydistance identical to the distance travelled by the light in the samplearm. The reflecting means may be a mirror or another structure havingreflective properties. An example may be a mirror mounted on a highprecision sledge system optionally including a piezo-electric elementcapable of vibrating, whereby for one position of the sledge a point maybe sampled many times.

Also the reflecting means may be means allowing variation of the opticalpath-length, such as a rotating mirror in the reference arm directingthe light field to a reflecting grating. As the mirror rotates thedistance to the grating changes and with this the optical path-length.

The length of the reference arm may be modulated by using apiezo-electric fibre stretcher, methods based on varying the path lengthof the reference arm light be passing the light through rapidly rotatingcubes or other rotating optical elements, and methods based onFourier-domain pulse-shaping technology which modulate the group delayof the reference arm light by using an angularly scanning mirror toimpose a frequency-dependent phase on the reference arm light afterhaving been spectral dispersed as discussed in U.S. Pat. No. 6,002,480which is hereby incorporated by reference.

In order to simulate the distance travelled by the light in the sample,the optical length of the reference path is preferably altered, and theapparatus according to the invention comprises means for altering theoptical length. The means for altering the optical path length may be anoptical modulator, for example an electro-optic modulator or a fibrestretcher.

Combining Means

The combining means is any suitable means capable of receiving two lightfields and combining the light fields into at least one light signal. Ina preferred embodiment the combining means is a coupler.

Detecting Means

The system comprises conventional detecting means. The detecting meansis essentially a photodetector chosen accordingly to match the sourcewavelength, a combination of photodetectors arranged to make up abalanced scheme, or a combination of photodetectors arranged to make upa double-balanced scheme.

Furthermore, the detecting means may be a linear array of photodetectorswithout or combined with a dispersive element arranged so that the arrayprovides depth and spectral information. The detecting means may also bea linear charge-coupled device (CCD) array without or combined with adispersive element arranged so that the array provides depth andspectral information.

Finally, the detecting means may be a two-dimensional array ofphotodetectors without or combined with a dispersive element arranged sothat the array provides depth and spectral information. The detectingmeans may also be a two-dimensional CCD array without or combined with adispersive element arranged so that the array provides depth andspectral information. For example, the dispersive element may be adiffraction grating (reflection or transmission), a prism or acombination of prisms.

End Reflections

In a preferred embodiment the SNR is further increased by reducingnon-sample reflections, such as the fibre end reflections in the samplearm. By reducing the non-sample reflections in combination withamplification of the first light field an increase of the relative SNRis increased up to about 20 dB, such as about 17 dB. It has been shownthat the amplification of the light field in the sample arm is improvedadditionally when reducing reflections.

The end reflections may be reduced by anti-reflex coating the fibre endsof the fibres in one or both of the arms.

Also the fibre ends may be cleaved at an angle to reduce reflections,such an angle being at least 5 degrees, such as preferably at least 7degrees.

The anti-reflex coating and the cleaving of the fibre ends may be usedas alternatives or in combination.

Processing/Displaying

The result obtained may be further processed to obtain relevantinformation based on the detection signal relating to thedistance/coherence. In one embodiment the detection signal is sent to acomputer for analysis. Depending on the object scanned, the computer mayprovide an image relating to for example the tissue scanned.

In relation to detection of inhomogeneities in for example opticalwaveguides, the computer may provide information relating to thedistance to the inhomogeity and for example also an image of theinhomogeneity.

The result may be sent from the computer to a display and/or a printerand/or stored in a storage means.

Penetration Depth

The parameters that govern OCT performance are longitudinal andtransverse resolution, dynamic range, measurement speed, and the centrewavelength of the light source.

The depth to which an illumination field of light penetrates withinturbid media, such biological tissue or the like, is determined by theamount of scattering and absorption present in the media.

In tissue scattering diminishes rapidly with increasing wavelengththroughout the visible and infrared wavelength regions. Absorption intissue is dominated by resonant absorption features, and no simplescaling can be assumed. For near-infrared light (˜0.8 μm), whereabsorption is relatively week, scattering is the dominant mechanism ofattenuation. At longer wavelengths, such as 1.3 μm , 1.55 μm or 1.9 μm,scattering is minimal, and water absorption becomes increasinglyimportant.

The longitudinal resolution governed by the coherence length isinversely proportional to the optical bandwidth of the light source.

By the present invention the penetration depths may be increased or evendoubled due to the increased SNR depending on the optical properties ofthe medium.

The transversal resolution is essentially given by the well-knowndiffraction limit, i.e. the minimum focal spot, which is the resolvingpower. The diffraction limit is determined by the wavelength, theeffective aperture of the beam and the focal length of the lens as knownfrom the art.

The measurement speed, i.e. the time to perform a single a-scan andcapture the interference signal, may be defined in different ways, andtherefore a unique measure for this quantity cannot be given. However,increasing the scan speed implies increasing the electrical bandwidth ofthe detecting means and this may ultimately lead to an increase of thereceiver noise. As shown by our analysis above, the introduction of theoptical amplifier amplifying the reflected light from the sample may beeven more advantageous if the noise in the detecting means increases. Inother words, the optical amplifier may to a certain extent aid toovercome receiver noise.

Thus, due to the amplification system according to the present inventionit is possible to conduct a faster scanning than with state of the artsystems.

Transverse Scanning

The light path preferably includes a transverse scanning mechanism forscanning the probe beam within the sample, for example an actuator formoving the apparatus in a direction substantially perpendicular to thesample. Such a scanning mechanism can have a micro-machined scanningmirror. A longitudinal scanning mechanism can also be provided to scanin a direction parallel to the probe beam. Scanning allows the apparatusto create images. Longitudinal scanning in the direction of the probebeam axis, along with scanning in a direction perpendicular to the axis,provides the possibility of obtaining an image of a vertical crosssection of the sample.

It is of course understood that although it is preferred to scan thesample apparatus in relation to the sample, the sample may also bescanned with respect to a stationary sample probe or a combination ofthese.

Applications

The apparatus and method according to the present invention may be usedin any application normally applying OCT scanning as well new technicalfields wherein the increased SNR allows the use of the presentapparatus. Thus, the apparatus may be used for so-called opticalbiopsies, wherein a segment of tissue, such as the skin, mucosa or anyother solid tissue is examined by OCT to diagnose any cellularabnormalities, such as cancer or cancer in situ. Furthermore, anymalignant growth may be detected by the present apparatus.

Due to the optical amplification conducted as discussed herein it ispossible to increase the relative SNR, for example 10 dB, such as about15 dB, drastically increasing the penetration depth of the system. Thus,above malignacies in the skin or mucosa may be detected directly byusing the present invention. Furthermore, the apparatus may be coupledto catheters or the like to scan internal body parts, such as thegastro-intestinal tract, a vessel or the heart or any body cavity. Also,the apparatus may be used for scanning during a surgical operation.

Also, the present apparatus has improved the use of OCT in ophtalmicalapplication due to the increased penetration depth, such as in cornealtopography measurements and as an aid in ophtalmical surgery, forexample for focusing on the posterior intraocular lens capsule for usein cataract surgery.

The present invention may also be applied in convention OLCRapplications, such as detection or imaging of inhomogeneities in opticalwaveguides or devices, i.e. wherein the sample is an optical waveguideor an integrated optical device. In another embodiment the sample may bea polymer. In yet another embodiment the sample may be a silicon-basedintegrated circuit.

EXAMPLES

Here follows a comparison of an OCT system according to prior art andtwo different OCT systems according to the present inventionexemplifying the added benefit of introducing an optical amplifier. Theeffect of using a system where all reflections contributing to I_(incoh)has been reduced as much as possible e.g. through coating of allsurfaces, as well as the effect of changing the splitting ratio on thesplitter from the source, is demonstrated. The former case in which allsurfaces are coated is reffered to as the coated case, whereas notcoating the surfaces is referred to as the uncoated case.

The system parameters used are $\begin{matrix}{{\text{〈}I_{source}\text{〉}} = {10\quad{mW},}} \\{{{\langle I_{noise}\text{〉}} = {2\quad{mW},}}\quad} \\{B = {10\quad{kHz},}} \\{\Gamma = {{r_{b\quad}{\exp\left( {{- \mu_{t}}z} \right)}} = {0.4\%\quad{\exp\left( {{- 5}\quad{{mm}^{- 1} \cdot 2}\quad{mm}} \right)},}}} \\{\Gamma_{noise} = \left\{ {\begin{matrix}{0.04,\quad{uncoated}\quad{case}} \\{10^{- 5},\quad{coated}\quad{case}}\end{matrix},} \right.} \\{{\text{〈}\Delta\quad i_{receiver}^{2}\text{〉}} = {{- 115}\quad{dBm}\text{/}{Hz},}} \\{\alpha = {0.9\quad A\text{/}W}} \\{\delta_{v} = {{c\left( {{1\text{/}\lambda_{\min}} - {1\text{/}\lambda_{\max}}} \right)} = {{c\left( {{1\text{/}1285\quad{nm}} - {1\text{/}1335\quad{nm}}} \right)} = {8.7\quad{THz}}}}}\end{matrix}$where the c is the speed of light in vacuum. The light source is chosento have a center wavelength of 1310 nm and a spectral bandwidth of 50 nm

Here a damping coefficient μ_(t)=5 mm⁻¹, a probing depth z=2 mm, and areflection coefficient within the sample of 4% have been chosen. Itshould be noted that more evolved and accurate methods to estimate rbased on the chosen parameters do exist in the art. However, for thepurpose of system performance evaluation, a rough estimate is sufficientto assign Γ a practical value.

Example 1a

a) Choice of Reference System

First, a reference system against which to compare the performance isdecided. This system can be seen in FIG. 5. Through a similar analysisas to the one used to derive Eq. 16′, the SNR of the reference systemmay be found. By doing so, it is found that the optimum splitting ratiotoward the sample is approximately 35% and 65% towards the referenceboth for a system with and without coated surfaces. A system with thissplitting ratio is chosen as reference system.

In the graphs in FIG. 6-FIG. 12, relative SNR implies that the systemunder investigation is compared to the corresponding reference system,which experiences the same conditions in terms of reflectivity, receivernoise etc. The novel system in FIG. 2 is compared to the referencesystem.

b) Optimum Splitting Ratio in the Absence of Amplification

First, the splitting ratio x is investigated in the absence of anamplifier for both the coated and uncoated case. The SNR for this systemis found from Eq (16) by letting χ=1 and I_(noise)=0. FIGS. 6 and 7shows the SNR of the system shown in FIG. 2 as a function of thesplitter ratio x relative to the reference system FIG. 3 when Γ_(noise)is in the uncoated and coated case, respectively.

From FIGS. 6 and 7 it is concluded that when an optical amplifier is notpresent in the system adjusting the splitting ratio away from 50/50 is adisadvantage.

c) Effect of Amplification for a Constant Splitting Ratio

Next, the effect of the amplifier is investigated over a wide range ofamplification factors and the splitting ratio is set to 50/50. FIG. 8shows the increase in SNR due to the use of an optical amplifier in thesystem shown in FIG. 2 for the uncoated case and FIG. 9 shows theincrease in SNR in the coated case. It is noted that the relative SNR isless than unity for low amplification factors. This is due to theamplifier having to compensate for the loss of optical signal power dueto the extra coupler in the system under investigation and addedamplifier noise.

d) Optimum Splitting Ratio for a Fixed Amplification

A conservative amplification factor of 100 (20 dB) is chosen and theeffect of choosing a different splitting ratio than 50/50 isinvestigated again. FIG. 10 and FIG. 11 shows the uncoated and thecoated case, respectively. These graphs demonstrate that in both casesit is an advantage to select a different splitting ratio then 50/50 fora system using optical amplification, and that the advantage of doingthis is highest in the coated case. It is also seen that the increase inthe relative SNR is about 400% higher for the coated case compared withthe uncoated case.

e) Optical Circulators and Fixed Amplification

Another realization of the novel OCT system is shown in FIG. 4, wherethe y-coupler in the sample part has been replaced with a so-calledoptical circulator known from the art. Obviously, the signal light poweris increased by a factor of four. Using the same parameters as above ind) for the coated case, the improvement in relative SNR for therealization in FIG. 4 is 44 compared to 12 for the realization in FIG. 2when both realizations are compared to the same reference system.

Finally, in FIG. 12 the sensitivity of the relative SNR on the receivernoise is demonstrated for the realization in FIG. 4. For a low thermalnoise the optical amplifier may be a disadvantage since the noise isdominated by the noise added by the optical amplifier. As the thermalnoise in the receiver is increased the optical amplifier becomes anincreased advantage because the optical noise added is gradually maskedby the thermal noise. For high values of the thermal noise the advantageof the optical amplifier is constant since the thermal noise is thedominant noise term.

Example 1b

a) Choice of Reference System

First, a reference system against which to compare the performance isdecided. This system can be seen in FIG. 5. Through a similar analysisas to the one used to derive Eq. 16, where the amplification has beenset to one and I_(noise) is set to zero, the SNR of the system may befound. By doing so, it is found that the optimum splitting ratio towardthe sample is approximately 35% and 65% toward the sample for the coatedand for the uncoated case. This is shown in FIGS. (14) and (15) whereΓ_(incoh) is in the uncoated and coated case, respectively. A systemwith this splitting ratio is chosen as the reference system.

In the graphs in FIG. 16-FIG. 22, relative SNR implies that the systemunder investigation is compared to the corresponding reference system,which experiences the same conditions in terms of reflectivity, receivernoise etc. The novel system in FIG. 13 is compared to the referencesystem.

c) Effect of Amplification for a Constant Splitting Ratio

Next, the effect of the amplifier is investigated over a wide range ofamplification factors and the splitting ratio is set to 50/50. FIG. 8shows the increase in SNR due to the use of an optical amplifier in thesystem shown in FIG. 2 for the uncoated case and FIG. 9 shows theincrease in SNR in the coated case. It is noted that the relative SNR isless than unity for low amplification factors. This is due to theamplifier having to compensate for the extra noise added by theamplifier, which for simplicity is assumed independent of amplificationratio.

d) Optimum Splitting Ratio for a Fixed Amplification

A conservative amplification factor of 100 (20 dB) is chosen and theeffect of choosing a different splitting ratio than 50/50 isinvestigated again. FIG. 10 and FIG. 11 shows the uncoated and coatedcase, respectively. These graphs demonstrate that in both cases it is anadvantage to select a different splitting ratio than 50/50 for a systemusing optical amplification although for chosen system parameters, theadvantage is only slight and the optimum splitting ratio is 45/55. It isseen that the relative increase in SNR by inclusion of an opticalamplifier is approximately a factor 5 higher in the coated case comparedto the uncoated case.

e) Optical Circulators and Fixed Amplification

Another realization of the novel OCT system is shown in FIG. (4), wherean optical circulator is inserted to avoid any reduction in the lightpower from the sample by the splitter. FIG. (12) and FIG. (13) shows theSNR of this system in the uncoated and coated case, respectively, whereall other parameters are the same as where used for FIG. (18) and (19).In both cases, it is an advantage to select a splitting ratiosignificantly different from 50/50, and the advantage of doing so isgreatest in the coated case. The advantage of inclusion of an opticalamplifier is seen to be a factor 9 higher in the coated case compared tothe uncoated case.

Finally, in FIG. 22 the sensitivity of the relative SNR on the receivernoise is demonstrated for the realization in FIG. 4. For a low thermalnoise the optical amplifier may be a disadvantage since the noise isdominated by the noise added by the optical amplifier. As the thermalnoise in the receiver is increased the optical amplifier becomes anincreased advantage because the optical noise added is gradually maskedby the thermal noise. For high values of the thermal noise the advantageof the optical amplifier is constant since the thermal noise is thedominant noise term.

The systems analyzed above should be considered typical examples.However, the advantage of introducing an optical amplifier is clearlypointed out. Firstly, it is demonstrated that the optical amplifier mayaid to overcome receiver noise leading to improved system performance interms increased SNR. The impact of optical amplification on an OCTsystem is highly dependent on the noise contribution from the receivingsystem, which comprises all components involved in obtaining anelectrical signal from the optical output e.g. electrical amplifiers,computer data collection system etc. This sensitivity is illustratedthrough FIG. 22, where the system with optical amplifier and opticalcirculator (shown in FIG. 4) is compared to the reference system in thecoated case. Secondly, an optimum splitting ratio different from 50/50has been demonstrated. Finally, adding an optical amplifier will be anincreased advantage as the electrical bandwidth of the receiver isincreased, which may lead to an increase of the receiver noise. In otherwords, the optical amplifier may to a certain extent aid to overcome theincrease in receiver noise. An increase in electrical detectionbandwidth is necessary when fast acquisition of measurement data desirede.g. for real-time imaging.

1. An apparatus for optical coherence reflectometry comprising a lightsource for providing a light signal splitting means for dividing saidlight signal into a first light field and a second light field, meansfor directing the first light field to a sample, and means for directinga first reflected light field from the sample, wherein an opticalamplifier is inserted in the first reflected light field, said opticalamplifier being different from the light source, and means for directingthe amplified first reflected light field to a combining means, so thatthe amplified first reflected light field is directed to the combiningmeans through another route than a route through the splitting means fordividing the light signal, means for directing the second light field toa reference path comprising a reflecting means, and means for directinga second reflected light field from the reference path to the combiningmeans, combining means for receiving said amplified first reflectedlight field and said second reflected light field to generate a combinedlight signal, and at least one detecting means for detecting thecombined light signal and outputting detecting signals.
 2. The apparatusaccording to claim 1, wherein the light source is a light emittingdiode, a super luminescent diode, other white light sources of suitablewavelength or a short-pulse laser.
 3. The apparatus according to claim1, wherein the optical amplifier is a semiconductor resonator,amplifier, resonant amplifier, fibre and/or Raman amplifier.
 4. Theapparatus according to claim 1, wherein the first light field isdirected to the sample without being amplified.
 5. The apparatusaccording to claim 1, wherein substantially all light energy from thefirst reflected light field is directed to the combining means.
 6. Theapparatus according to claim 1, wherein substantially all light energyfrom the second reflected light field is directed to the combiningmeans.
 7. The apparatus according to claim 1, wherein the reflectingmeans of the reference path is a mirror or a retroreflector.
 8. Theapparatus according to claim 1, comprising means for altering theoptical length of the reference path.
 9. The apparatus according toclaim 1, wherein the splitting means is bulk-optic, fibre optic or ahologram.
 10. The apparatus according to claim 1, wherein the splittingratio of the splitting means is substantially 50%/50%.
 11. The apparatusaccording to claim 1, wherein the splitting ratio of the splitting meansis changeable, so that from 1% to 99% of the light energy from the lightsource is directed to the sample arm.
 12. The apparatus according toclaim 11, wherein more than 50% of the light energy is directed to thesample.
 13. The apparatus according to claim 1, wherein two detectingmeans are arranged to obtain a balanced detection signal.
 14. Theapparatus according to claim 1, wherein at least one CCD camera isarranged as a part of the detecting means to detect a part of the firstreflected light field.
 15. The apparatus according to claim 1, whereinat least a part of the means for directing the first light field is anoptical fibre.
 16. The apparatus according to claim 15, furthercomprising means for reducing non-sample reflection.
 17. The apparatusaccording to claim 16, wherein the fibre-ends are anti-reflectioncoated.
 18. The apparatus according to claim 16, wherein the fibre-endsare cleaved at an angle.
 19. The apparatus according to claim 18,wherein the angle is at least 5 degrees.
 20. The apparatus according toclaim 1, further comprising an actuator means for moving the apparatusin a direction substantially parallel to the sample.
 21. The apparatusaccording to claim 1, further comprising an actuator means for movingthe apparatus in a direction substantially perpendicular to the sample.22. The apparatus according to claim 1, further comprising processingmeans for providing a result of the detection signals.
 23. The apparatusaccording to claim 22, further comprising a display device displayingthe result from the processed detection signals.
 24. A method forproviding a result of a sample comprising establishing a light sourcefor providing a light signal, splitting said light signal into a firstlight field and a second light field, directing the first light field toa sample, and the second light field to a reference path, receiving thefirst reflected light field from the sample, optically amplifying thefirst reflected light field, receiving the second reflected light field,combining said amplified first reflected light field and said secondreflected light field to generate a combined light signal, detecting thecombined light signal obtaining detection signals, and processing thedetection signals obtaining the result image of the sample.
 25. Themethod according to claim 24, wherein the sample is skin or mucosa. 26.The method according to claim 24, wherein the sample is retina.
 27. Themethod according to claim 24, wherein the sample is a vessel or heart.28. The method according to claim 24, applied during a surgicaloperation.
 29. The method according to claim 24, wherein the sample is apolymer.
 30. The method according to claim 24, wherein the sample is asilicon-based integrated circuit.
 31. The method according to claim 24,wherein the sample is an integrated optical device.
 32. The methodaccording to claim 24, wherein the sample is an optical waveguide. 33.The method according to claim 24, wherein the light source is a lightemitting diode, a super luminescent diode, other white light sources ofsuitable wavelength or a short-pulse laser.
 34. The method according toclaim 24, wherein the optical amplifier is a semiconductor resonator,amplifier, resonant amplifier, fibre and/or Raman amplifier.
 35. Themethod according to claim 24, wherein the first light field is directedto the sample without being amplified.
 36. The method according to claim24, wherein substantially all light energy from the first reflectedlight field is directed to the combining means.
 37. The method accordingto claim 24, wherein substantially all light energy from the secondreflected light field is directed to the combining means.
 38. The methodaccording to claim 24, wherein the reflecting means of the referencepath is a mirror or a retroreflector.
 39. The method according to claim24, comprising means for altering the optical length of the referencepath.
 40. The method according to claim 24, wherein the splitting meansis bulk-optic, fibre optic or a hologram.
 41. The method according toclaim 24, wherein the splitting ratio of the splitting means issubstantially 50%/50%.
 42. The method according to claim 24, wherein thesplitting ratio of the splitting means is changeable, so that from 1% to99% of the light energy from the light source is directed to the samplearm.
 43. The method according to claim 42, wherein more than 50% of thelight energy is directed to the sample.
 44. The method according toclaim 24, wherein two detecting means are arranged to obtain a balanceddetection signal.
 45. The method according to claim 24, wherein at leasta part of the means for directing the first light field is an opticalfibre.
 46. The method according to claim 45, further comprising meansfor reducing non-sample reflection(s).
 47. The method according to claim46, wherein the fibre-ends are anti-reflection coated.
 48. The methodaccording to claim 45, wherein the fibre-ends are cleaved at an angle.49. The method according to claim 48, wherein the angle is at least 5degrees.
 50. The method according to claim 24, further comprising anactuator means for moving the first light field in a directionsubstantially parallel to the sample.
 51. The method according to claim24, further comprising an actuator means for moving the first lightfield in a direction substantially perpendicular to the sample.
 52. Themethod according to claim 24, wherein the wavelength of the light sourceis in the range from 500 nm to 2000 nm.
 53. The method according toclaim 24, wherein the wavelength of the light source is in the rangefrom 1250 nm to 2000 nm.
 54. The method according to claim 24, whereinthe wavelength of the light source is in the range from 600 nm to 1100nm.
 55. An apparatus for optical coherence reflectometry comprising: alight source for providing a light signal, splitting means for dividingsaid light signal into a first light field and a second light field,means for directing the first light field to a sample, means fordirecting a first reflected light field from the sample, and means forreducing fibre-end reflection, wherein an optical amplifier is insertedin the first reflected light field, said optical amplifier beingdifferent from the light source, and means for directing the amplifiedfirst reflected light field to a combining means, so that the amplifiedfirst reflected light field is directed to the combining means throughanother route than a route through the splitting means for dividing thelight signal, means for directing the second light field to a referencepath comprising means for altering the optical path length, and meansfor directing a second light field from the reference path to thecombining means, combining means for receiving said amplified firstreflected light field and said second reflected light field to generatea combined light signal, and at least one detecting means for detectingthe combined light signal and outputting detection signals.